The present invention relates to a solder bump forming method suitably applied to the manufacture of a semiconductor device, in which a barrier metal such as a BLM (Ball Limiting Metal) film is used to relocate a solder bump, e.g., a solder ball bump in a region different from a region on an electrode pad, and more particularly to a solder bump forming method and an apparatus for carrying out the method wherein in flip chip mounting a semiconductor device chip on a mounting substrate through a solder bump relocated on a metal film, e.g., a barrier metal deposited on an undercoating formed of an organic compound such as polyimide, the reliability on bonding strength of the solder bump and mechanical strength and electrical characteristics of a product device assembled by flip chip mounting can be improved.
For further miniaturization of electronic equipment, increasing a components mounting density is an important point. Also regarding a semiconductor IC chip, a high-density mounting technique such as a flip chip mounting method for mounting an LSI bare chip directly on a printed wiring board rather than a related art package mounting method is increasingly developed at present.
As the flip chip mounting method, various methods including an Au stud bump method and a solder ball bump method are used. In any methods, a barrier metal is interposed between an Al electrode pad of a semiconductor IC and a bump material, so as to improve adhesion between the electrode pad and the bump and prevent mutual diffusion of metal components.
In the solder ball bump method, the barrier metal has an effect on the finished shape of the bump, so the barrier metal is usually called a BLM (Ball Limiting Metal) film.
As the structure of the BLM film in the solder bump method, a three-layer structure of Cr/Cu/Au is most generally used. The Cr film as a lower layer functions mainly as an adhesion layer to the Al electrode pad. The Cu film as an intermediate layer functions mainly as an antidiffusion layer for preventing diffusion of solder metal components. The Au film as an upper layer functions mainly as an antioxidation film for preventing oxidation of the Cu film.
A related art solder ball bump forming method includes the steps of depositing a BLM film on an Al electrode pad of an LSI chip, pattering the BLM film, depositing a solder metal film composed mainly of Pb and Sn on the BLM film, and melting the solder metal film by heat treatment to deform it into a ball, thus forming the solder ball bump on the electrode pad.
FIGS. 1A to 1E show a related art method of forming a solder ball bump as a bonding portion for a flip chip IC on an electrode pad by using a lift-off process for a photoresist film and a vacuum evaporation process for a solder metal.
As shown in FIG. 1A, an Al electrode pad 82 of Al-Cu alloy, for example, is formed on a semiconductor substrate 81 of silicon, for example, by sputtering and etching. Then, a surface protective film 83 of polyimide or silicon nitride, for example, is formed over the entire surface of the substrate 81. A first opening 84 is next formed through the surface protective film 83 so as to expose the electrode pad 82, and a multilayer metal film of Cr/Cu/Au, for example, as a BLM film 85 is formed on the Al electrode pad 83 including a side wall of the first opening 84.
As shown in FIG. 1B, a resist pattern 87 having a second opening 86 larger in diameter than the first opening 84 is formed on the BLM film 85.
As shown in FIG. 1C, a solder evaporated film 88 is deposited over the entire surface of the substrate by a vacuum evaporation process.
As shown in FIG. 1D, the solder evaporated film 88 on the resist pattern 87 is removed together with the resist pattern 87 by a lift-off process for a photoresist film to leave the solder evaporated film 88 on the Al electrode pad 82.
As shown in FIG. 1E, the solder evaporated film 88 is melted by heat treatment to form a ball-shaped solder ball bump 89 on the Al electrode pad 82 through the BLM film 85.
In the above method, a process flow till the pattern formation of the BLM film 85 shown in FIG. 1A by use of a lift-off process of a photoresist film will now be described in detail with reference to FIGS. 2A to 2D.
As shown in FIG. 2A, the surface protective film (passivation film) 83 is deposited on the Al electrode pad 82 formed on the semiconductor substrate 81, and a first opening 90 having a predetermined size as a connection hole is formed through the surface protective film 83. Then, a photoresist film 91 is deposited over the substrate, and next patterned to form a second opening 92 larger in diameter than the first opening 90 of the passivation film 83.
Then, the wafer having a layered structure shown in FIG. 2A is set in a plasma processing device to perform pretreatment prior to deposition of the BLM film 85 (usually called back-sputtering) by RF plasma. As a result, the side wall of the second opening 92 of the photoresist film 91 is deformed into an overhanging shape to reduce the diameter of the second opening 92 at its opening edge 93 as shown in FIG. 2B.
In the next step, the multilayer film of Cr/Cu/Au is deposited as the BLM film 85 by sputtering on the substrate. As shown in FIG. 2C, the BLM film 85 is not deposited on an overhanging side wall surface 94 of the resist pattern 91 formed by the RF plasma pretreatment mentioned above, so that the BLM film 85 deposited on the substrate is separated into a part deposited on the Al electrode pad 82 and a part deposited on the photoresist film 91.
In the next step, the wafer having a layered structure shown in FIG. 2C is immersed into a resist removing liquid and oscillated with heat. As a result, the BLM film 85 deposited on the photoresist film 91 is lifted off together with the photoresist film 91, thereby forming a pattern of the BLM film 85 connected to the Al electrode pad 82 through the first opening 90 as the connection hole (corresponding to the first opening 84 shown in FIG. 1A).
As described above, in most cases according to the related art solder ball bump method, solder ball bumps are formed on only the electrode pads located in the periphery of the LSI chip.
However, in a future LSI chip with the microstructure of the semiconductor device chip being advanced and the distance (pitch) between the adjacent electrode pads being increasingly reduced, the solder ball bumps formed on the adjacent electrode pads may come into contact with each other to cause an electrical short circuit in the above related art method. If the bump diameter is reduced to avoid the contact between the adjacent solder ball bumps, it become difficult to maintain a bonding strength between the LSI chip and a printed wiring board, resulting in a reduction in reliability of mechanical connection and electrical connection.
As means of avoiding the contact between the adjacent solder ball bumps, a method of relocating the bumps on a region of the LSI chip different from a region on the electrode pads has been adopted. This method is schematically shown in FIG. 3, for example. As shown in FIG. 3, an additional bump forming region B is provided at a position different from an electrode pad A, and a solder ball bump C is formed on the bump forming region B. Further, any wiring D is formed between the electrode pad A and the bump forming region B.
If the bump forming region B and the wiring D for relocation of the bump can be formed from the BLM film, the related art method can be utilized without increasing the number of steps only by changing a mask pattern for the photoresist film for the lift-off process. Accordingly, no additional processing devices are required to have a great advantage from the viewpoints of cost and productivity.
In this respect, the following process flow has been developed. The outline of this process flow will now be described with reference to FIGS. 4A to 4G.
As shown in FIG. 4A, an Al electrode pad 104 is provided on a semiconductor substrate 102, and a silicon nitride film 106 as a surface protective film is next deposited over the entire surface of the substrate 102. Further, a first opening 108 is formed through the silicon nitride film 106 so as to expose the Al electrode pad 104.
As shown in FIG. 4B, a first polyimide film 110 is deposited over the entire surface of the substrate 102, and patterned to form a second opening 112 smaller in diameter than the first opening 108 of the silicon nitride film 106 at a position over the Al electrode pad 104.
In the next step, a photoresist film 114 is deposited over the entire surface of the substrate and patterned to form a third opening 116 for exposing the Al electrode pad 104, a solder ball bump forming region, and a wiring forming region connecting the Al electrode pad 104 and the solder ball bump forming region. Subsequently, a BIM film 118 is deposited by sputtering over the entire surface of the substrate. Prior to sputter deposition of the BLM film 118, the photoresist film 114 is subjected to back-sputtering to thereby reduce the diameter of the third opening 116 at its opening edge, so that the BLM film 118 is deposited separately on a region corresponding to the third opening 116 and on the photoresist film 114 as shown in FIG. 4C.
As shown in FIG. 4D, the photoresist film 114 is lifted off to remove the BLM film 118 on the photoresist film 114 together with the photoresist film 114, thereby forming a BLM rewiring portion 120 connected to the Al electrode pad 104 and having the solder ball bump forming region different from a region on the Al electrode pad 104, and the wiring region between the Al electrode pad 104 and the solder ball bump forming region.
As shown in FIG. 4E, a second polyimide film 122 is deposited over the entire surface of the substrate and patterned to form a fourth opening 124 for exposing the solder ball bump forming region.
In the next step, a lift-off process for a photoresist film is carried out as similarly to the related art method shown in FIGS. 1B to 1D to form a solder evaporated film 126 composed mainly of Pb and Sn at the fourth opening 124 as shown in FIG. 4F.
Further, as similarly to the related art method, the solder evaporated film 126 is melted by heat treatment to form a ball-shaped solder ball bump 128 on the BLM film 118 as shown in FIG. 4G.
However, in the actual solder ball bump forming step according to the above process flow, there arises a new problem such that a bonding strength of the solder ball bump is low. This problem is due to the fact that since most of the undercoating for the BLM film is the first polyimide film 110 of an organic compound, the adhesion at the interface between the BLM film and the underlying polyimide film becomes weaker than the adhesion at the interface between the BLM film and the underlying Al electrode pad in the related art structure as shown in FIG. 1E.
As a result, the strength of a bump bonding portion of a product assembled by flip chip mounting a semiconductor device chip on a printed wiring board through the solder ball bumps by the above process flow becomes low to cause a reduction in reliability of the mechanical strength and electrical connection of the product set and also exert an adverse effect on durability.